The bottom sediments of aquatic habitats are rich in decaying organic matter from both plant leaves and stems and the bodies of everything from plankton to fish. These sediments are home to countless small invertebrate animals, themselves feeding on the sediments or hiding among them. Although sometimes thought of as nothing more than dirt by aquarists, this detritus and the animals it contains represent one of the most concentrated food resources available to fish. Many species use it and some, known as detritivores, specialize in extracting food from it.

Can fish really eat dirt?

Analysis of many specie's diets has revealed that detritus plays a major part in fish nutrition. Detritivory is especially common during the low-water season of many tropical rivers, including the Orinoco and Zambezi, when alternative foods become progressively harder to find. At these times of year, the edible sediments consist mostly of decaying plant matter. Although it sounds unappetizing, this still has a reasonable amount of value as a food, not only for the vegetable matter, but also for the community of tiny protozoans such as Paramecium that live on it. Even during more plentiful times of year, detritus still forms a significant part of the diet of omnivorous fish.

How ore fish adapted to feeding from the substrate?

Detritivores are extremely common in tropical systems and the way they turn over and recycle the nutrient has a major effect on the ecosystems they live in. Not only that, but these fish form the prey of larger fish, so a whole food web is connected by these vital creatures. Although few of the major detritivores of the Amazon have found popularity in the aquarium trade, a large number of omnivorous species that sift the detritus are commonly found, including the Corydoras group of catfish and Geophagus cichlids, whose name translates as “earth-eaters.” Fish that feed at the substrate tend to have mouths at the end of their snouts, very often positioned low down on the head, or even beneath it. Often, the eyes of these fish are set back, away from the mouth, which prevents them from being damaged as the fish digs for food. When feeding on detritus, fish take large quantities of food into their mouths and sift it, spitting out and discarding small stones and indigestible items. Often, the fish have a large number of tightly packed gill rakers, which act like a sieve, allowing the water through, but catching the microscopic food particles before they escape.

If detritus is a natural part of aquatic ecosystems, why is it so essential to filter it from the home aquarium?

Sediments and detritus in the home aquarium can spell trouble for all the inhabitants because the decaying matter can be a source of nitrites. What is more, it may appear unsightly. It is therefore understandable that aquariums are usually equipped with power filters to remove all hints of dirt before they have the chance to accumulate. However, it would be rewarding for anyone with a large aquarium to attempt a more naturalistic set-up, where low fish stocking densities allow for a filter-less system. Under these conditions, a layer of sediment would accumulate, feeding the plants and fish alike and creating a true aquatic ecosystem.